The morphological changes in histochemically demonstrable glycogen were evaluated following 5 minute bilateral carotid occlusion in gerbils. The first appearance of abnormal increase in glycogen granules was observed in hippocampus after 2 hours following 5 minute ischemia. The accumulation of glycogen in astrocytic cells reached its peak at 6 hours after release of carotid occlusion. This was followed by a striking reduction in glycogen, especially in hippocampus, which was observed at 24 hours. A maximal accumulation of glycogen was conspicuous in Schaffer's collaterals at 48 hr post-ischemic time interval. These observations indicate that periods of previously demonstrated neuronal hyperactivity are associated with a conspicuous reduction of glycogen, whereas a collapse of neuronal activity corresponds to a conspicuous accumulation of glycogen, mainly in astrocytic cells. The morphological observations on glycogen provide thus an insight into changes in energy metabolism in cerebral ischemia and they contribute to a better understanding of this so clinically and important condition.